The Cyprus Institute’s Pentakomo Field Facility (PFF) has finished succesfully the first major experiment by the development of a pilot/experimental facility for the co-generation of electricity and desalinated seawater from CSP. Specifically, the experimental plant consists of a heliostat-central receiver system for solar harvesting, thermal energy storage in molten salts followed by a Rankine cycle for electricity production and a multiple-effect distillation (MED) unit for desalination.
PFF is a major infrastructure for research, development and testing of technologies relating to concentrated solar power (CSP) and solar seawater desalination. It is located at the south coast of Cyprus near the sea and its environmental conditions are fully monitored. It provides a test facility specializing in the development of CSP systems suitable for island and coastal environments with particular emphasis on small units (<25 MWth) endowed with substantial storage, suitable for use in isolation or distributed in small power grids.
Solar thermal systems are one of the most promising modalities of Renewable Energy Sources in regions endowed with high values of Direct Normal Irradiation (DNI). Concentrating the solar radiation leads to high temperatures on the receiver, and therefore to the potential of high thermodynamic efficiency. Additionally, thermal energy readily lends itself to storage, thus making Concentrated Solar Power (CSP) a lead candidate for providing dispatchable energy from a renewable source.
A heliostat field was designed and built in collaboration with the Commonwealth Scientific and Industrial Research Organization (CSIRO) of Australia employing CSIRO’s proprietary focusing heliostat design. The field layout was optimized to maximize annual energy yield and minimize shading. The field consists of 50 heliostats, each with a reflective area of 5 m2 and constructed out of a single mirror facet. Each mirror has a reflectivity of 93% and was pre-stressed to form a paraboloid of revolution.
The central receiver is placed on a 14 m tower. The receiver is a cavity type receiver with a circular aperture of 0.8 m in diameter – more details on the receiver are given in the following section.
Figure 2 Simplified schematic of the plant layout, with the following components schematically indicated: 1) the heliostat field, 2) the central receiver, 3) the molten salt storage tank, 4) the steam engine and 5) the MED desalination unit.
Steam and Electricity Production
Steam and Electricity Production Steam and electricity production units were added to the experiment to demonstrate a viable path towards the cogeneration of electricity and desalinated seawater.. A forced circulation steam loop was designed, circulating water in a heat exchanger immersed in the molten salt to create saturated steam.
The currently employed steam turbine was an impulse from Green Turbine 1.5 kW that operated on superheated steam at pressure values between 1.3 bar-g and 4 bar-g and a maximum steam temperature of 200 °C. In the nex major project a Green Turbine 15 kW will be used.
The exhaust steam from the turbine is used as thermal input to the desalination, while the remaining energy from the steam is used to preheat the seawater for the desalination process. In regard to thermal energy for driving the Rankine cycle, a heat exchanger consisting of a pair of coils was designed to preheat water from room temperature to 200 °C (saturated liquid state) and then steam at a temperature between 270 °C to 500 °C, depending on the solar salt temperature.
The use of the buffer tank was therefore critical in ensuring a stable thermodynamic state of the steam within the design thermodynamic parameters of the engine. A superheated coil in contact with the thermal energy storage TES lid was.
FIGURE 5 shows a detailed layout of this steam loop, which is in essence a The A forced circulation steam loop was designed, circulating water in a heat exchanger immersed in the molten salt to create saturated steam cycle design combined with water desalination. A 10kWth/1.5kWe steam engine was procured and a custom 4-effect MED unit was developed and constructed for the co-generation of electricity and desalinated seawater, respectively.
Ray tracing simulation of the field was performed using SolTrace to determine the average annual power delivered by the field to the receiver aperture.ne of the aims of the PFF demonstration facility is to show the feasibility of co-generation of electricity and desalinated seawater on a continuous (24/7) basis. A single-tank thermal energy storage (TES) systems using molten salt as the heat storage medium was designed and constructed in collaboration with the Molten solar salt was chosen as both the heat transfer fluid and thermal storage medium, after considering the operational temperature range of the mixture, its volumetric heat capacity, as well as economic considerations.
The tank has a height of 2.8 m and volume of 8 m3 , and is designed to operate at temperatures up to 600 °C in a non-pressurized environment, resulting in a total thermal storage capacity up to 0.6 MWh.
The new field facility for solar research at Pentakomo Cyprus, has become recently operational and it offers a unique environment for testing in realistic coastal – island conditions solar technologies, in particular for electricity production and solar desalination.
The next major experiment (CSP-DSW) to be conducted at the PFF is the testing of the Cogeneration of Electricity and Desalinated Sea Water using Concentrated Solar Power. This experiment, if succesful is intented to lead to the construction of a pilot facility (in the range of 2 to 8 MW), which should be economically viable .
Italian National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA-Casaccia).